Dr. J. E. Gray on a new genus of Mytilicbe. 63 



the sinuosity of the lower edge, in the entire absence of any small 

 teeth under the umbo, and in the paleaceous periostraca. 



1. STAVELIA TORTA. 



Mytilus tortus et M. horridus, Dunker, Proc. Zool. Soc. 1856 ; 

 Reeve, Conch. Icon. t. 3. f. 6 & 9. 



Hab. North Australia and Philippines. 



I cannot discover any permanent character between the two spe- 

 cimens described by Dr. Dunker. 



The specimens of this genus in the Museum, and others which 

 have come under my observation, offer a peculiarity which I have 

 hitherto only observed in a very few other bivalve shells, and in none 

 to the extent which is presented in this species. 



In my paper " On the Formation and Structure of Shells," in 

 the * Philosophical Transactions' for 1833 (reprinted by Dr. John- 

 ston, t Letters on Conchology,' p. 413), I observe, — 



" In some very rare instances the shells (bivalves) are also reversed ; 

 but the fact is not easily observed except in the unequal-valved kinds. 

 There were formerly in the Tankerville collection two specimens of 

 Lucina Childreni, in one of which the right valve was a dextral 

 shell, in opposition to the general structure. These specimens are 

 now in the British Museum Collection." 



The four specimens of this shell which I have under my eye pre- 

 sent the same anomaly as the two specimens of Lucina Childreni 

 above referred to, that is to say, two of them have the left valve the 

 flattest and furnished with the large lobe on the front of the ventral 

 margin, and in the other two it is the right valve which has this 

 form and development ; and I cannot observe any other peculiarity 

 in the specimens except this indifference between the development 

 of the sides of the animal. So that, as in Lucina Childreni, it 

 is impossible to determine which is the normal form of the species. 

 A somewhat similar indifference as to the direction of the shell is to 

 be observed in some land univalve shells, as Bulimus aureus, where 

 the shell appears to be indifferently dextral and sinistral ; but in the 

 genus Stavelia it appears more extraordinary on account of the great 

 difference of the form of the two valves. 



"We have just received from China a large species of Muteladce, 

 allied to Unio Grayii of Lea, (which I do not name, as Mr. Cuming 

 informs me that Mr. Isaac Lea is describing and figuring it in Phila- 

 delphia*), which offers a curious peculiarity. 



These shells have the hinder extremity twisted up on one side 

 somewhat like Area tortuosa, but not so regularly ; and unlike that 

 species, the flexure is not always in the same direction : some have 

 the bend towards the right, and the others towards the left of the 

 animal. 



I may observe, that, as far as I have been able to examine, the side 

 seems a matter of indifference, for as many of the specimens are 

 bent to the one side as the other. 



* Triquetra lanceolata seu contorta, Lea. 



